The invention relates to a current source and sink, and more particularly to a precision tracking, switchable bipolar current source/sink with ground clamping (zeroing) at the discrete circuit level.
A variety of constant current devices and circuits are known in the art. The fundamental textbook constant current circuit is a constant voltage source series connected to a load through a high impedance (usually resistive) device. This type of device has several limitations. It generally requires high voltages with high power dissipation in the resistor. Also, the current is not readily programmable or controllable over a range by means of another voltage. To overcome these problems, transistors may be used, taking advantage of the base-emitter voltage (V.sub.be) match of two or more bipolar transistors (e.g., current mirrors, Wilson mirrors and extensions) or the pinch mode operation of field effect transistors (FET's). These implementations are programmable and fairly compliant, but are practical only when used on an integrated circuit where transistor characteristics can be closely matched.
One discrete device solution is the constant current diode, which is essentially a FET with its gate tied to source or a pair of cross-coupled FET's. Another very practical, adjustable, compliant and often-used current sink places the base-emitter junction of a transistor into the feedback loop of a operational amplifier. Unfortunately, tight regulation at low current usage is poorly controlled due to operation near cut-off. Errors are especially noted with thermal variations.
Currently, common current sources and sinks use the variable impedance of an active semiconductor device in conjunction with a fixed voltage to vary the output current depending on load conditions in an effort to stabilize the current to some preset value. However, since semiconductor impedance devices of the type described are polarity sensitive, these devices may act as current sources or sinks, but not both.